This invention relates to a phosphate-containing supported chromium catalysts for olefin polymerization.
Supported chromium catalysts can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Silica supported chromium catalysts can also be used to prepare olefin polymers in a slurry system wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has the advantage of being less complex. However certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form process. For instance, in the solution process, control of the molecular weight can be effected by changing the temperature with lower molecular weight (higher melt flow) being obtained at the higher temperature. However, in the slurry process, this technique is inherently limited since any efforts to increase the melt flow to any appreciable extent by increasing temperature would cause the polymer to go into solution and thus destroy the slurry or particle-form process. It is known to extend the range of melt flow capability of a given catalyst through the use of hydrogen. However, it has not heretofore been commercially feasible to produce a complete spectrum of polymers, so far as melt flow is concerned, in a slurry system with a single chromium catalyst system.
Also it is frequently desired to have a polymer with broader molecular weight distribution than is normally obtained in the slurry or particle-form process. Different applications require polymer having different flexural modulus or stiffness. While it is frequently desired to have a stiffer polymer than is normally produced, there are also many applications where it would be desirable to have a lower flexural modulus.
There would be certain advantages to utilizing zerovalent chromium in supported chromium olefin polymerization catalyst systems in that there are some problems associated with calcining a base already containing chromium. However, while it has long been known to utilize zerovalent chromium on silica supports, such use has never achieved commercial success because of the very low activity associated therewith. While it has long been known to use metal phosphates as supports for chromium olefin polymerization catalysts, such supports have never achieved commercial success. Thus olefin polymerization using the chromium catalyst system is still carried out utilizing hexavalent chromium on a silica-containing base in a manner similar to that used decades ago when the chromium catalyzed polymerization of olefin polymers first became commercial.